Solid attacker against Snorlax, Rhydon and Tyranitar.
Specially against Tyranitar, since you will resist every possible moveset from him and will take advantage from his double weakness to Fight.
Also good against Blisseys, but you will have some trouble with the big ones.
I would keep him at level 20 (I assume it has around 1400CP with 100% IVs and this is a good range to prestige against Tyranitar), but if you want to power him up to 30, it'll also be useful.